282 



Scientific Proceedings, Royal Dullin Societjj. 



plasticine. These jars were placed in front of a window and suitably shaded, 

 so that they wo'e illuminated from one side onlj^ In some cases the leaves 

 were left intact, and in others the leaflets were cut off where they joined 

 the petiole. The amount of bending was deduced by taking measurements of 

 the horizontal displacement of the top of the petiole. 



As a result of a number of experiments, it was found that the petioles 

 from which the leaflets had been removed responded to phototropic stimuli as 

 readily as those of intact leaves. The power of response of the decapitated 

 petioles, however, was lost after a period of one to two days. An experiment 

 in Avhich both the growth in length and the amount of bending in the intact 

 and decapitated petioles were measured, showed that the loss of the power 

 of response in the decapitated petioles could be correlated with cessation of 

 growth. In all cases the region of greatest growth and bending was about 

 4-6 cm. from the top of the petiole. 



In order to obtain evidence regarding the vexed question as to whether 

 phototropic stimiili can be perceived by the lamina and transmitted to the 

 petiole, experiments were carried out on Sparmannia africana. This plant 

 was found to be very suitable for the purpose as the leaves readily set them- 

 selves in the optimum position relative to incident light. The larger leaves are, 

 on an average, about 1.5 cm. in length by 12 cm. broad, being supported 

 on petioles abotit 12 em. in length. 



a 



Fig. 1. 



Plant of Sparmannia africana, viewed from above, showing tlie methods adopted in 

 shadiuff the leaves. 



Two plants were chosen which were growing in pots and had been exposed 

 to light from above in a greenhouse, with the result that the leaf-blades were 

 all horizontal. In each plant a pair of similar leaves lying close together' 

 m the same horizontal plane was selected. A piece of cardboard was supported 

 by means of a clamp attached to a retort stand just above, but not touching, 

 the adjacent edges of the two leaves in the manner shown in fig. 1, a. The 

 pots containing the plants were standing on the floor of a greenhouse, and were 

 surrounded by tall screens so that they were illuminated from above only. 

 Under these circumstances the shades affected small portions only of the leaf- 

 blades, and were without influence on the light falling on the petioles. 



